When designing a communication apparatus for a motor vehicle, the problem often arises of the transceiver unit and the antenna having to be arranged at different installation locations. Therefore, a relatively long cable is needed to communicate the transmission signals from the transceiver unit to the antenna. The signal attenuation of the cable is therefore noticeable when emitting the transmission signals. Some communication standards provide a prescribed transmission power for the transmission signals to be emitted. The actual power with which a transmission signal is emitted by the antenna may be unknown even though it has been generated with a known signal power at a transmission output of the transceiver unit because it has been attenuated by the cable.
An amplifier device can amplify the transmission signal again at the antenna end of the cable in order to compensate for the cable attenuation. In such systems, it is necessary to know the transmission gain value by which the transmission signal must be amplified by the amplifier device. One possibility for determining the transmission gain value involves measuring the power of the transmission signals once at the transceiver end of the cable and once at the antenna end of the cable. The transceiver measuring circuit, however, must be wired to the control device for the amplifier device. This results in a large amount of wiring and connection complexity when providing the communication apparatus.